This invention relates to display devices. More specifically, this invention relates to electrochromic display devices which incorporate tractable, stable, conductive polymers as the display material.
Electrochromic devices are display devices which incorporate materials having a persistent but easily reversible color change. Such changes are effected in the material by an applied electric current. The original color is regained upon reversing this process. Good electrochromic display devices hold their color and charge in an open circuit situation.
These reversible color changes can be accomplished with either of two broad categories of solid: the non-polymeric type and the polymeric type. Included in the non-polymeric type are the metal oxides such as lithium salts of tungsten trioxide, iridium oxide, and the like. Another type of non-polymeric material is the small organic molecules capable of undergoing electrically produced reversible oxidation/reduction reactions in which the two states have different colors, e.g., viologen compounds and the like. A third type of non-polymeric electrochromism is the electrically induced deposition or dissolution of a thin film of a metal, e.g., silver. Such devices have a high power requirement and are slow to write and erase.
The polymeric electrochromic displays include those in which electrochromic groups are bound to a backbone polymer such as polystyrene. They are similar to the non-polymeric devices in that they require high power and are slow to write and erase. The other polymeric electrochromic displays are those in which the polymer contains monocyclic heterocyclic rings in the polymer backbone such as polypyrrole or polythiophene. These latter two types are fast to write and erase and need only lower power to charge and discharge. However, these electrochromic polymers are intractable and difficult to fabricate. It is difficult to synthesize and control the morphology of polypyrroles and polythiophenes. The result is a polymer which at high molecular weights cannot be fabricated, and at low, solvent soluble molecular weights has poor solid characteristics.
Thus, it would be highly desirable to have an electrochromic display material which is tractable and easy to work with and fabricate in any desired shape. In addition, it is also desirable to have a device which requires low power and possesses a fast write/erase speed. It would also be desirable for the displays to be multicolor processable.